A novel cellular structure in the photoreceptors of insectivorous birds

Abstract

The keen visual systems of birds have been relatively well-studied. The foundations of avian 21 vision rest on their cone and rod photoreceptors. Most birds use four cone photoreceptor types 22 for color vision, a fifth cone for achromatic tasks, and a rod for low-light levels. The cones, 23 along with their oil droplets, and rods are conserved across birds -with the exception of a few 24 shifts in spectral sensitivity -despite taxonomic, behavioral and ecological differences. Here, 25 however, we describe a novel photoreceptor in a group of New World flycatchers (Empidonax 26 spp.) in which the traditional oil droplet is replaced with a complex of electron-dense 27 megamitochondria surrounded by hundreds of small, orange oil droplets. These photoreceptors 28were unevenly distributed across the retina, being present in the central region (including in the 29 fovea), but absent from the retinal periphery and the area temporalis. Many bird species have 30 had their oil droplets and photoreceptors characterized, but only the two flycatchers described 31here (E. virescens and E. minimus) possess this unusual structure. We discuss the potential 32 functional significance of the unique sub-cellular structure in these photoreceptors providing an 33 additional visual channel for these small predatory songbirds. 34

Mapping the avian visual tectofugal pathway using 3D reconstruction

“…The layers of the retina consist of the ganglion cell, the inner and outer plexiform, and the inner and outer nuclear layers. Avian retinae have at least six classes of photoreceptors (chicken: Kram et al, 2010;Okano et al, 1992;López-López et al, 2008;pigeon: Bowmaker, 1977;goose: Moore et al, 2012;starling: Hart et al, 1998;blue tit: Hart et al, 2000;zebra finch: Bowmaker et al, 1997;emu: Hart et al, 2016; however, see Tyrrell et al, 2019) which selectively respond to visual stimuli. These photoreceptors are distributed in a regular mosaiclike pattern in the retina and consist of four spectrally different single cones, double cones, and rods (Kram et al, 2010).…”